Is there a critical outlet size for clogging of granular silos
In plain words
Grains pouring through a small hole in a silo eventually jam by forming an arch. It is disputed whether there is a hole size above which jamming never happens, or whether jamming only becomes extremely rare.
Precise statement
For grains of diameter $d$ discharging through an orifice of diameter $D$, the mean number of grains $\langle s\rangle$ discharged between clogs grows with $D/d$. Determine whether $\langle s\rangle$ diverges at a finite critical $D_c/d$ as (D_c - D)^(-gamma), or grows exponentially in a power of $D/d$ with no true transition, in 2D and 3D silos.
What would settle it
Measurements or simulations of $\langle s\rangle(D/d)$ over enough decades to separate a power-law divergence from exponential growth, supported by a theory of arch formation.
Status in the literature
Unverified note
Thomas and Durian (2015, https://doi.org/10.1103/PhysRevLett.114.178001) found exponential growth with no critical outlet size, contradicting the earlier power-law divergence of Zuriguel et al. (2005); the issue is unresolved in 3D and for frictional grains (2026).